Thermal transfer apparatus providing transfer control

ABSTRACT

Heat pipe apparatus comprising a heat pipe having a first section having spiral fins extruded on its inner surface and a second section having a smooth bore interior. The pipe is sealed at each end after evacuation of all noncondensable gases and contains a vaporizable liquid as the medium for heat transfer. The second section of the pipe has a U-shaped portion which, when liquid filled, acts as a vapor or liquid trap when suspended vertically to prevent heat transfer in either direction. Since no capillary return of condensate is possible through the smooth bore section of the pipe when the trap is in an open position, unilateral heat transfer toward either end of the pipe is made possible.

liite ttes oberts, .lr.

atet [1 1 Sept. 24, 1974 THERMAL TRANSFER APPARATUS I PROVIDING TRANSFERCONTROL [73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

22 Filed: Nov. 9, 1973 21 Appl. No.2 414,506

UNITED STATES PATENTS 3,543,839 12/1970 Shlosinger 165/105 X 3,613,77410/1971 Bliss, Jr [65/105 X 3,688,838 9/1972 Sturm et al. 165/105 X HEATTRANSFER AREA A 3,700,028 10/1972 Noren [65/105 X PrimaryExaminer-Albert W. Davis, Jr.

Attorney, Agent, or Firm-W. H. Kamstra [57] ABSTRACT Heat pipe apparatuscomprising a heat pipe having a first section having spiral finsextruded on its inner surface and a second section having a smooth boreinterior. The pipe is sealed at each end after evacuation of allnoncondensable gases and contains a vaporizable liquid as the medium forheat transfer. The second section of the pipe has a U-shaped portionwhich, when liquid filled, acts as a vapor or liquid trap when suspendedvertically to prevent heat transfer in either direction. Since nocapillary return of condensate is possible through the smooth boresection of the pipe when the trap is in an open position, unilateralheat transfer toward either end of the pipe is made possible.

5 Claims, 1 Drawing Figure BACKGROUND OF THE INVENTION adapted toaccomplish such heat transfer action by means of closedevaporation-condensation cycles.

Heat pipes adapted to transfer heat away from one area to another areaof heat dissipation have long been known in the art. Because of theirhigh efficiency in transferring relatively large quantities of heat withsmall temperature gradients, heat pipes have been particularly useful,for example, in the temperature control of electronic equipment. Atypical prior art heat pipe operates in the following manner. A sealedchamber which, although it may take various forms, for present purposesis assumed to be tubular, has evacuated therefrom all noncondensablegases. At the area from which heat is to be transferred, e.g., a printedwiring circuit board, a fluid within the tube at that point is in itsliquid state at the normal operating temperature of the circuit boardelectronics. The interior of the tube normally contains for its entirelength a wick which comprises a cylindrical cluster of fine wire or, asis contemplated in the present invention, the wick may comprise aradially extending, equally spaced plurality of fins spirally extrudedon the interior wall of the tube. The free space within the tubecontains only the vapor of the fluid at a pressure which corresponds tothe saturation pressure of the fluid at the aforementioned'operatingtemperature of the circuit board electronics.

Should for some reason the operating temperature of the circuit boardand hence the temperature of the tube at that point, rise from thenormal, the liquid boils--at a very small temperature change since thepressure within the tube is very nearly zero. The resulting vapor is ata substantially higher pressure and will migrate toward the cooler endof the tube where it condenses on the cooler surfaces of the tube walland wick. As the vapor condenses it gives off the heat acquired earlierduring vaporization. The extruded fins on the interior of the tube aredimensioned so that the condensate is conveyed back to the heat sourcearea by capillary action where the cycle is repeated until the normaloperating temperature of the circuit board is restored. The efficiencyof a heat pipe arrangement may be appreciated from the fact that it isable to conduct many hundreds of times as much heat as an excellentconductor such as a copper rod.

In many heat pipe applications it is found advantageous to control thedirection of heat transfer within the heat pipe and even to interruptthe transfer altogether. Seasonal temperature changes, for example, maydictate a discontinuance of the heat transfer; interruption in heattransfer may also be required in the temperature testing of electroniccircuitry or other apparatus, to name another example. Electronic orother equipment may require protection from momentary heat surgeswithout a reduction of its normal operating temperature.

It is accordingly an object of this invention to control heat transferand heat transfer direction in a heat pipe cooling arrangement.

SUMMARY OF THE INVENTION The foregoing and other objects of thisinvention are realized in one illustrative embodiment thereof comprisinga heat pipe formed of a two section tube hermetically sealed at thecoupling of the two sections and at the ends of the integral structurethus formed after evacuation of all noncondensable gases. The innersurface of one section has spirally extruded or otherwise formed thereona plurality of radial, inwardly extending fins. The inner surface of theother section is smooth. The two section structure is longitudinallycoaxial; however, the smooth bore section has formed therein a U-shapedportion extending from the longitudinal axis. Although the interior ofthe tube is very nearly a vacuum, an amount of fluid in liquid form isenclosed within the tube sufficient to fill the base of the U- shapedportion with the tube virtually horizontal and the latter portionsuspended downwardly. The pipe assembly is rotatably mounted at oppositeends, permissibly at a slight inclination, by frictional bearing meansto permit its at least partial rotation to fixed angular positions.

In one mode of operation, the heat pipe assembly is rotated to positionthe U-shaped portion substantially downwardly with the result that, bygravitational action, the enclosed fluid in liquid form is collected inthe base of the latter tube portion. The quantity of liquid isdetermined as being sufficient to fill the entire lower cross-section ofthat tube portion with the result that it acts as a trap to preventmovement of vapor or liquid from either end of the heat pipe to theother. The unavailability of any capillary action in the smooth borepermits this trapping action and no heat transfer is thus possible. Inanother mode of operation the 'pipe assembly is rotated to position theU-shaped portion sufficiently horizontally to permit the containedliquid under gravitational force to flow in along the lower inner wallportion of the pipe thereby lowering its level to permit in turn thefree movement of vapor in either direction along the heat pipe. The pipeis positioned with both its wick section and its condenser section atindividual heat source areas. Assuming the temperature at the wicksection to be higher than that of the source of the condenser section,the heat at the former point vaporizes the liquid which conventionallymigrates to the condenser section where the vapor gives off its heat ascondensation occurs. The inner wall of the pipe at this end is smooth sothat the condensate is returned to the wick section by gravitationaldrainage rather than by capillary action as is the case in conventionalheat pipe arrangements.

Should the temperature of the heat source at the condenser end of thepipe be higher than that of the source at the wick section, no heattransfer to the latter section can take place since no liquid is nowavailable at that point for evaporation. The unavailability of capillaryaction in that direction prevented any prior return of condensate to thegreater heat area. This is in contrast with known heat pipe arrangementsin which the transfer of heat between two sources would permit thetemperature at the area of the lower temperature heat source to rise tothe additive level of both that source and the heat transferred from thehigher temperature heat source.

It is accordingly a feature of this invention that in a heat pipeassembly, a wick section is combined with a smooth bore section topermit an open and a closed vapor switch action by means of a trapsection and also to make possible unilateral heat transfer between twoheat sources.

BRIEF DESCRIPTION OF THE DRAWING The objects and features of thisinvention will be better understood from a consideration of the detaileddescription of the organization and operations of one illustrativeembodiment thereof when taken in conjunction with the single FIGURE ofthe accompanying drawing depicting the structure, in partial cutawayform, of a heat pipe arrangement according to the principles of thisinvention.

DETAILED DESCRIPTION A heat pipe arrangement according to this inventionis shown in the drawing as comprising a first and a second pipe sectionand 11. The sections are cut away to expose their inner surfaces; pipesection 10 has spirally extruded or otherwise formed on its innersurface a plurality of inwardly and radially extending fins 12,representative ones of which are shown. The inner surface of pipesection 12 is smooth and the section has formed therein a substantiallyU-shaped portion 11 which, in the drawing, is shown as suspendeddownwardly in its position to function as a vapor trap in the manner tobe described. The two pipe sections 10 and 11 are coupled coaxiallylongitudinally by any convenient coupling means 13 which also serves asa seal. The ends of the pipes are capped by hermetically sealing cappingmeans 14 and 15, the latter means of which may be knurled to provide aconvenient grip for rotating the pipe assembly about its axis. Thelatter rotation may be accomplished in suitable friction bearing andsupport means such as the exemplary supports 16 and 17 shown in thedrawing. Other mounting arrangements and structure may be envisioned asdetermined by particular installation requirements.

Before finally capping the pipe ends, all noncondensable gases areevacuated from the assembly by means well known in the art and a smallamount offluid 18 in liquid form is introduced. A number of materialsare available in the art for performing a coolant function; the onechosen will be determined by the coolant requirements of the apparatusto be cooled and will, of course, be vaporized by the heat of theapparatus to be dissipated. The fluid 18 is shown in the drawing ascollected in pipe section 10 as the result ofa slight inclination ofthepipe assembly. A heat pipe arrangement according to this inventionconstructed as thus described may typically be used to function in anumber of operative modes in relation to a heat source or sources fromwhich the transfer of heat is to be controlled.

In each of the operative modes, assuming the heat source to be locatedat the wick end, or section 10, of the pipe assembly indicated in thedrawing as heat transfer area A, heat generated at the latter areacauses the vaporization of the fluid 18. Heat absorbed by thisvaporization is carried by the vapor as it migrates to the condenser endof pipe section 11 indicated in the draw ing as heat transfer area B.Conventionally, the heat is there dissipated by condensation if thetemperature at that point is lower than that at area A. Assuming thiscondition, in a conventional prior art heat pipe arrangement, thecondensate is returned to pipe section 10 by capillary action. Inaccordance with the present invention, such capillary return isprevented by the smooth bore of pipe section 11. In one mode ofoperation of this invention, any return of the condensate is preventedaltogether by a rotation of the heat pipe assembly so that the U-shapedportion 11 of pipe section 11 is suspended downwardly as depicted in thedrawing. The longitudinal axis of the pipe assembly is contemplated asbeing arranged at a slight inclination toward the wick end, say, of theorder of 2, for example. The condensate will in this case, undergravitational action, accumulate in the vapor trap formed by the U-shaped portion 11'. The amount of fluid 18 originally introduced in thepipe assembly is determined as sufficient at least to fill completelythe lower pipe crosssection of pipe portion 11. Any further circulationof vapor or liquid in either direction is as a result effectivelyprevented. Has the heat pipe assembly been provided with wick means forits entire length as is the case in conventional practice, capillaryaction would have permitted a return of the condensate despite the trap.To restore vapor circulation and heat transfer, the pipe assembly isrotated in its friction bearing supports 16 and 17 substantially ineither direction to a horizontal position, one of which is indicated bythe dashed outline in the drawing. In this position another mode ofoperation is advantageously possible.

With the trap portion 11 in its open position, vapor from the condensersection is now free to return to the wick section 10 without, however,as mentioned in the foregoing, the assistance of capillary action. Theonly manner of return available is a gradual drainage undergravitational action. This action is furthered by the slight inclinationof the assembly. With the pipe assembly in its position permitting freevapor circulation, a balancing of heat transfer between two heat sourcesis also possible. In this operative mode, a first and a second heatsource are assumed to be located at heat transfer areas A and B,respectively. In a state in which the temperature of the source at areaA is greater than that of the source at area B, normal heat transferwill take place in the direction from area A to area B. At the latterpoint, condensation will conventionally occur and the condensate willreturn to the wick section 10 at area A under gravitational action.However, should the temperature of the source at area B rise equal to orabove that of the source at area A, no condensation will take place atthe condenser section at area B. Since the smooth bore and slightinclination of pipe section 11 prevented any movement of condensate fromthe now low temperature end either by capillary or gravitational action,there was no fluid at the high temperature end at area B to evaporate.Heat transfer from the latter area to area A is thus again effectivelyprevented. Heat transfer will resume when the temperature of the sourceat area B again falls below that of the source at area A. This operationcontrasts with the different result encountered by a prior art heat pipearrangement under the same operating conditions. In the prior art case,condensate would return to the higher temperature area B via capillaryaction. As a result, to the temperature prevailing at area A due to theheat source there located will be added the heat transferred from area Bby the conventional evaporation-condensation process. In the heat pipearrangement of the present invention, the heat source at area B can makeno such contribution to the temperature at area A.

A heat pipe arrangement according to this invention, exemplaryoperations of an illustrative embodiment of which were described in theforegoing, is advantageously useful for cooling electronic apparatus ina telephone central office, for example. Heat transfer area A would, insuch an application, be positioned within the office building. The pipesection 11 is then extended through the building wall with the heattransfer area B exposed to the elements outside of the office building.If the building is unheated and unattended, for example, during a coldseason the heat generated by the telephone equipment may be dissipatedin the lower ambient temperature of the office building interior. Inthis case, the additional cooling provided by the heat pipe may not berequired and the assembly is ro tated to set the trap portion 11downwardly in its closed position as shown in the drawing. On the otherhand, the temperature of the office building interior may rise to apoint where additional cooling of the equipment is required. The pipeassembly is then rotated so that the trap portion 11 is in its openposition to permit free vapor circulation and condensate return is bygravitational action. Should the temperature at the condenser end of theheat pipe at the exterior of the building now rise above that at theequipment end inside of the office building due to sunlight, forexample, no vaporization is possible at the now hotter condenser end ofthe pipe due to the absence of condensate at that end. The exteriorlygenerated heat can accordingly not be transmitted back in the pipe toadd to the heat being generated by the office electronic equipment.

In the drawing the various structural elements of the invention areshown out of the proportions they would assume in its practice for thesake of clarity. Accordingly, it will be appreciated that in aparticular application the pipe assembly may be considerably longer withrespect to its diameter than as shown inthe drawing, for example. Itwill be further understood that what has been described is considered tobe only one illustrative heat pipe arrangement according to theprinciples of this invention and it is to be further understood thatvarious and numerous other arrangements may be devised by one skilled inthe art without departing from the spirit and scope of the invention asdefined by the accompanying claims.

What is claimed is:

1. Thermal transfer apparatus comprising a heat pipe means having acapillary evaporator section comprising spiral fins formed on the innersurface of a first portion of said pipe means, a condenser sectionhaving a substantially smooth inner surface and having a substantiallyU-shaped bend in said last-mentioned portion, said pipe means beingsealed at each end, a vaporizable liquid contained within said pipemeans, and means for rotatably supporting said pipe means at a slightinclination from the horizontal.

2. Thermal transfer apparatus comprising a heat pipe means having anevaporator section including wick means therein, a condenser sectionhaving a substantially smooth inner surface and having a substantiallyU-shaped bend formed therein, said pipe means being sealed at each end,and a vaporizable liquid exclusively contained within said pipe meanssufficient to fill a cross-section of said bend.

3. Thermal transfer apparatus as claimed in claim 2 in which said wickmeans comprises a plurality of spiral fins radially extending on theinner surface of said evaporator section.

4. Thermal transfer apparatus as claimed in claim 3 in which said heatpipe means is arranged at a slight inclination from the horizontal alongits longitudinal axis, said pipe means being mounted to permit rotationof said bend at least between a downwardly position and a substantiallyhorizontal position.

5. Thermal transfer apparatus comprising a sealed heat pipe means havingan evaporator section including wick means therein, a condenser sectionhaving a substantially smooth inner surface, vapor trap means comprisinga U-shaped bend formed in said condenser section, a vaporizable liquidexclusively contained within said pipe means sufficient to fill across-section of said bend, and means for rotatably mounting said heatpipe means at a slight inclination from said condenser section to saidevaporator section, said vapor trap means being in a closed positionwhen rotated substantially downwardly and being in an open position whenrotated in either direction substantially horizontally.

1. Thermal transfer apparatus comprising a heat pipe means having acapillary evaporator section comprising spiral fins formed on the innersurface of a first portion of said pipe means, a condenser sectionhaving a substantially smooth inner surface and having a substantiallyU-shaped bend in said lastmentioned portion, said pipe means beingsealed at each end, a vaporizable liquid contained within said pipemeans, and means for rotatably supporting said pipe means at a slightinclination from the horizontal.
 2. Thermal transfer apparatuscomprising a heat pipe means having an evaporator section including wickmeans therein, a condenser section having a substantially smooth innersurface and having a substantially ''''U''''-shaped bend formed therein,said pipe means being sealed at each end, and a vaporizable liquidexclusively contained within said pipe means sufficient to fill across-section of said bend.
 3. Thermal transfer apparatus as claimed inclaim 2 in which said wick means comprises a plurality of spiral finsradially extending on the inner surface of said evaporator section. 4.Thermal transfer apparatus as claimed in claim 3 in which said heat pipemeans is arranged at a slight inclination from the horizontal along itslongitudinal axis, said pipe means being mounted to permit rotation ofsaid bend at least 90* between a downwardly position and a substantiallyhorizontal position.
 5. Thermal transfer apparatus comprising a sealedheat pipe means having an evaporator section including wick meanstherein, a condenser section having a substantially smooth innersurface, vapor trap means comprising a U-shaped bend formed in saidcondenser section, a vaporizable liquid exclusively contained withinsaid pipe means sufficient to fill a cross-section of said bend, andmeans for rotatably mounting said heat pipe means at a slightinclination from said condenser section to said evaporator section, saidvapor trap means being in a closed position when rotated substantiallydownwardly and being in an open position when rotated in eitherdirection substantially horizontally.